I'm also going to move this thread from the FileMaker Community Feedback Space (which is specifically for input on the Community itself) to the Discussions Space where you should receive more views and potentially more feedback on this topic!

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If you're sure you can email Yahoo securely from, say, a regular email client, what I would do in this situation is EXTERNALIZE the email to a mini web service that uses the extremely powerful, but easy-to-use JavaMail API.

I've done this several times with FileMaker. The person using the FileMaker application has no idea that email is handled any differently as all they do is click a button (or tie the email to an event or whatever) to send the email.

Assuming you're using the same settings, and can do secure emails externally but not using FileMaker, then there is a problem with FileMaker, not Yahoo!

Personally, with all the published data hacks, I avoid Yahoo!, but that's not always possible.